This invention relates to the field of electrical keyboards. More particularly, this invention relates to the field of membrane keyboards wherein a stable operational pressure or actuation force may be achieved regardless of the size of the switch operational area.
Membrane keyboards of the general type with which this invention is concerned are well known in the art. These keyboards conventionally have a pair of circuit layers, one fixed and one movable, separated by a spacer or separator layer. The circuit layers are sheets of insulating material, with circuit patterns thereon. These circuit patterns face each other and are separated by a spacer, which has apertures at the location of aligned contact elements on the fixed and movable circuit sheets. Electrical switching is effected by applying finger or other pressure to specific locations on one of the circuit sheets to move a contact on that circuit sheet through an aperture to make contact with a contact element on the other circuit sheet. The fixed and movable circuit layers and the spacer may be separate sheets of material, or any two or three of those sheets may be formed from a single sheet of material folded over in any desired fashion. Keyboards of this configuration are generally formed in a laminate construction with the layers bonded together, sealed or otherwise fixed against relative lateral movement between the layers. The assembly may also include an overlay sheet with indicia of one kind or another to identify key locations and a backer plate to support the assembly.
Membrane keyboard switches as hereinabove described are generally provided with a decorative overlay sheet which is mounted on the first circuit layer or switch sheet to assist the keyboard operator in proper key selection. The size of each particular switch operational area will often differ from key to key as will the corresponding switch motion cavities underneath each key. Thus, larger key areas have larger corresponding switch motion cavities. Unfortunately, larger switch motion cavities need less actuation force or pressure than smaller cavities. This is an undesirable situation for the keyboard operator because of the great disparity in operational pressure from key to key and the relatively large forces needed to actuate small key areas.